Environmental factors associated with the decline of dwarf inanga Galaxias gracilis McDowall in New Zealand dune lakes

Abstract

ABSTRACT1. The dwarf inanga Galaxias gracilis is restricted to 13 dune lakes along an 80 km length of coastline on the west coast of the North Island, New Zealand, and its abundance has declined in at least eight of these lakes. It is therefore a rare and endangered endemic species.2. A range of environmental variables were compared between lakes where it is still common and lakes where it is now rare to identify variables associated with its low abundance.3. Lakes where dwarf inanga were rare all had a lower pH, lower concentrations of calcium, magnesium and silica, and higher concentrations of chloride than lakes where dwarf inanga were common. In addition, they contained different charophyte plant species, as well as stocked populations of eels and/or introduced exotic fish species capable of preying on one or more of the life stages of dwarf inanga.4. Factors not associated with the low abundance of dwarf inanga included lake size and depth, land use in the catchment, lake level fluctuations, the extent of emergent vegetation, littoral zone substrate composition, and water quality parameters including maximum water temperature, water transparency, and minimum oxygen levels in the hypolimnion.5. The identification of factors associated with the low abundance of dwarf inanga has allowed testable hypotheses to be developed to account for its decline in certain lakes. However, biological data, particularly on the location of spawning grounds and on the feeding requirements of larvae, are required before all these hypotheses can be tested.6. The results have formed the basis for a management plan for the conservation of this species, in which whole-lake experiments are being carried out to identify the effects of introduced plants and fish on its population size.

Agonistic interactions between Gambusia affinis and Galaxias maculatus: implications for whitebait fisheries in New Zealand rivers

“…Gambusia has gained a global reputation as a pest fish species and has been implicated in the reduction or extinction of small native fish in a wide range of locations (Myers, 1965; Meffe et al., 1983; Arthington and Lloyd, 1989; Courtenay and Meffe, 1989; Lloyd, 1990; Marsh and Minckley, 1990; Lydeard and Belk, 1993; Howe et al., 1997; Gill et al., 1999; Rinco et al., 2002; Mills et al., 2004; Caiola and de Sostoa, 2005). In New Zealand, gambusia is a potential competitor with, and predator of, black mudfish, Neochanna diversus (Barrier and Hicks, 1994), and is primarily responsible for the rarity of the endemic dune lakes galaxias, Galaxias gracilis , in three Northland dune lakes, including its extinction in one of them (Rowe and Chisnall, 1997; Rowe, 2003). However, Ling (2004) reviewed the limited knowledge of its impact in New Zealand and predicted that its direct effect on native fish through competition and predation would be minimal because it occupied an under‐utilized niche (i.e.…”

Sensitivities of mosquitofish and black mudfish to a piscicide: Could rotenone be used to control mosquitofish in New Zealand wetlands?

“…Mosquitofish have been implicated in the extinction or severe reduction of native fish species overseas (Myers 1965;Meffe 1985;Lloyd 1990;Arthington 1991), and can modify habitats by significantly altering plankton community composition (Hurlbert et al 1972;Hurlbert & Mulla 1981). In New Zealand, mosquitofish have been implicated in the decline of the dwarf inanga, Galaxias gracilis (Rowe & Chisnall 1997;Rowe 1998), and have been observed eating mudfish fry (Barrier 1993;Barrier & Hicks 1994). Large mosquitofish will chase and nip juvenile mudfish (Barrier & Hicks 1994), and preliminary results suggest that the growth of adult mudfish is reduced in the presence of mosquitofish (authors' unpubl.…”

Creating and destroying species: the ‘new’ biodiversity and evolutionarily significant units among New Zealand's galaxiid fishes

“…Although, some of these landlocked populations have been described as separate species in the past, for example, Galaxias usitatus McDowall, from northern North Island, they are all now recognized as simply ecomorphotypes of the parent species (McDowall, 1972). The dwarf inanga is threatened by loss of habitat, habitat modification and introduced fishes, and further extinction of some lake populations is likely (Rowe & Chisnall, 1997b). Although the species occupies a very small geographic range, it was necessary to determine whether separate populations represented evolutionarily significant units for conservation, and also to establish affinities with catadromous and landlocked inanga populations in New Zealand.…”